1. Field Of The Invention:
The present invention relates to a method of manufacturing bodies of silicon nitride and to a silicon nitride body produced by the method.
2. The Prior Art:
Silicon nitride, chemical formula Si.sub.3 N.sub.4, is a ceramic material which has been of considerable interest in recent years as a feasible material for the construction of components which operate at high temperatures and in a corrosive atmoshperes. Contrary to most other ceramic materials having high strength, its ability to withstand thermal shock is excellent, because of its low coefficient of thermal expansion.
Applications which are of particular interest for silicon nitride include construction material for turbine wheels, rotor blades and other dynamically stressed parts for gas turbines, including gas turbines for vehicle operation, as a construction material for parts in Wankel engines as well as for a bearing material.
Products of silicon nitride are manufactured in two well known ways, either by hot-pressing or by reaction bonding.
Hot-pressed products are made are made by compressing silicon nitride powder together with sintering-promoting additives, generally magnesium oxide, in a graphite tool at temperature of about 1700-1800.degree. C. and at a pressure which may reach 20-30 MPa. The quantity of magnesium oxide added is 0.8-5% of the weight of the silicon nitride. For hot-pressing a silicon nitride is used which at least substantially consists of silicon nitride of .alpha.-phase type and which thus only contains a small quantity of silicon nitride of .beta.-phase type, or possibly none at all. Unlike silicon nitride of .beta.-phase type, silicon nitride of .alpha.-phase type contains small quantities of oxygen in the molecules. During the hot-pressing, the magnesium oxide reacts to form magnesium silicate and silicon nitride of .alpha.-phase type is converted to silicon nitride of .beta.-phase type. The formation of magnesium silicate is thought to occur because of the action of silicon dioxide which exists as a thin coating on the silicon nitride grains and also because of the action of the oxygen present in the silicon nitride of .alpha.-phase type. Generally the silicate also contains a certain amount of calcium. The magnesium silicate forms a glasslike binding phase between the silicon nitride particles. It is known that the magnesium oxide is of decisive importance for the compression and sintering process in hot-pressing and that it assists in forming a dense, sintered product which is extremely strong. It is also known that the magnesium silicate phase in the sintered product causes a reduction in the strength of the product at higher temperatures, such as temperatures of 800-900.degree. C. and above, a reduction which does not occur with reaction-sintered silicon nitride. In order to improve the strength at high temperatures, experiments have been made using oxides other than magnesium oxide as the additive, for instance BeO, Ce.sub.2 O.sub.3, Y.sub.2 O.sub.3 and La.sub.2 O.sub.3. It has been found that a certain improvement could be achieved with the use of Y.sub.2 O.sub.3 or BeO instead of MgO, but that the reduction in strength at higher temperatures is still considerable.
Reaction-bonded silicon nitride is manufactured by producing a porous body of silicon powder which is then nitrided with nitrogen gas. The body can easily be shaped prior to nitridization, and the shape changes very little during this process. The porosity of the finished product exceeds 15%. Unlike products made from hot-pressed silicon nitride, the strength of products made from reaction-bonded silicon nitride is practically independent of temperature, at least up to temperatures of 1600.degree. C., but the strength is considerably less than for hot-pressed silicon nitride. Reaction-bonded silicon nitride cannot therefore be used for highly stressed construction components.